Multi-bladed fans

ABSTRACT

A variable pitch fan gas turbine engine has a multi-bladed fan including a rotor and a plurality of fan blades. The fan blades are mounted in pairs on circular root members by means of pin root fixings, and the root members are rotatable about radial axes through their centres to vary the angle of attack of the blades.

United States Patent [191 McMurtry et al.

[451 Jan. 14, 1975 MULTI-BLADED FANS [75] Inventors: David Roberts McMurtry; Derek Percival Harris, both of Bristol, England [73] Assignee: Rolls-Royce (1971) Limited,

London, England 22 Filed: June 20, 1973 21 Appl. No.: 371,687

[30] Foreign Application Priority Data July 6, 1972 Great Britain 31570/72 [52] U.S.Cl 416/120,416/13l,416/121 [51] Int. Cl. F0ld 7/00 [58] Field of Search ..416/120,121,l31,135, 416/200, 212

[56] References Cited UNITED STATES PATENTS 2,344,266 3/1944 Reissner 416/141 X 2,436,087 2/1948 Benson 416/140 2,587,247 2/1952 Turner 416/200 2,936,155 5/1960 Howell et a1 416/134 2,944,787 7/1960 Gingras 416/140 2,990,156 6/1961 Marshall et a1. 416/140 3,202,343 8/1965 Emmermann et a1 416/132 UX FOREIGN PATENTS OR APPLICATIONS 93,963 7/1962 Denmark 416/198 302,953 12/1928 Great Britain.. 416/140 606,151 8/1948 Great Britain 416/131 621,315 4/1949 Great Britain 416/131 711,703 7/1954 Great Britain 416/212 916,310 l/1963 Great Britain 416/198 A Primary Examiner-Everette A. Powell, Jr. Attorney, Agent, or Firm-Stevens, Davis, Miller & Mosher 57 ABSTRACT A variable pitch fan gas turbine engine has a multibladed fan including a rotor and a plurality of fan blades. The fan blades are mounted in pairs on circular root members by means of pin root fixings, and the root members are rotatable about radial axes through their centres to vary the angle of attack of the blades.

1 Claim, 5 Drawing Figures PATENTED JAN 1 4|975 SHEET 0F 4 MULTI-BLADED FANS The present invention relates to muIti-bladed fans and relates in particular to a construction of multibladed fans for ducted fan gas turbine engines.

It is known to provide a bladed rotor for a gas turbine engine in which two aerofoil blades are provided on a single root which is supported on a rotor disc. With such blades, however, the roots have been of the dove tall or fir tree type and a disadvantage of these types of roots is that they require correspondingly shaped notches to be cut in the rotor disc rim for engagement with the blade roots. These notches can act as crack starters under heavy loadings of the rotor.

According to the present invention a multi-bladed fan comprises a rotor having a plurality of fan blades, a plurality of circular root members and means for mounting at least two of the fan blades on each one of the root members, said means including pivot means whereby the fan blades are capable of pivoting towards and away from each other.

By this means the use of notches in the disc is avoided. Also the stresses in the root members can be reduced compared to the known arrangements, and the bending stresses in the blade due to centrifugal and gas loading on the blade can be reduced. This latter advantage particularly applies where the fan blades are for use in a variable pitch fan engine.

In one form of the invention the circular blade roots are carried by bearings in a rotor so as to be capable of rotating about the root axis to vary the angle of attack of the blades.

In a preferred form of the invention the blades are carried on a rotor which comprises two annular side walls which are of undulating form so that at their periphery they alternately meet and are spaced apart thereby providing a plurality of angularly spaced apart rings defining apertures for receiving the circular blade roots.

An embodiment of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

FIG. 1 illustrates a ducted fan engine incorporating a variable pitch multi-bladed fan of the present invention,

FIG. 2 is a sectional elevation through the disc and blades of the variable pitch fan of the engine of FIG. 1.,

FIG. 3 is a sectional elevation on the line III-III of FIG. 2, and

FIGS. 4 & 5 are plan views of a set of rotatable fan blades in the normal cruise position, and a reverse thrust position respectively.

Referring now to the drawings in FIG. 1 there is shown a variable pitch ducted fan gas turbine engine comprising a core engine 1 which is arranged to drive a fan 2 which in turn is situated in a duct 3 defined between an outer cowl 4 and a nose bullet 5. The core engine may be of any desired type since the invention is basically concerned with the fan itself and the type of core engine is immaterial to the invention.

In such a fan engine it is desirable to minimise the hub-tip ratio to keep the frontal area down, and this is a restriction on the number of blades which can be mounted on the rotor because the sizes of the roots become smaller as the hub decreases in diameter so that the stress levels in the roots become excessive.-

This is overcome by the present invention by putting two blades on one circular root. The area of the root is thus greatly increased. For example, if the root diameter for the double blade arrangement is made twice the diameter of an equivalent circular root for a single blade, the area of the root of the double blade is four times that of the single blade. Thus the stress levels are halved for the same number of blades.

From FIGS. 2 and 3 it will be seen that each blade 10 is attached to its circular root 11 by a pin root fixing which comprises a pin 13 passing through segments 14 of the blade and segments 15 of the circular root alternately. Each pin is retained by a head 16 at one end and a nut 16a at the other end, which also serve to retain structure 35, later described to the blade.

The circular blade root 11 is supported for rotation about its axis 17, which lies in a radial direction relative to the centre line of the engine, by bearings 18 mounted in the rotor 19, thus the pitch of the fan blades, i.e., their angle of attack, may be varied by rotating the root 11. The rotor 19 comprises two annular side walls 20, 21 which are of undulating form so that at their periphery they alternately meet and are spaced apart thereby providing a plurality of angularly spaced apart rings defining apertures for receiving the circular blade roots 11 and the bearings 18.

Each blade root is assembled into its respective aperture by loosely assembling the radially outer race 25, the bearings 18 and the radially inner race 26, sliding the root 11 through the centre of the assembly and rotating the inner race so that it screws onto the blade root along the screw thread 27 until a flange 28 on the inner race abuts the root.

The outer race is then screwed into the aperture in the disc by the screw thread 29. A shaft 30, connected at its radially inner end to a pitch change mechanism (not shown) has splines 31 at its radially outer end which engage splines on the circular root for rotating the root about its axis 17.

The splines on the shaft are extended beyond the length necessary to engage the circular root and also engage a lock washer 32. This lock washer 32 is connected to the inner race 26 by dogs 33 to prevent the screw thread 27 coming loose. A flange 34 on the shaft prevents the circular root assembly dropping through the outer race whilst the rotor is stationary. In operation centrifugal force acting on the blades applies a large radially outward load to the bearings and takes up the clearances provided therein.

The circular blade roots are surrounded by a sheet metal structure 35, part of which rotates with the fan and part of which is stationary. In FIG. 2 which is a sectional view, a boss 12 is indicated which is formed in the sheet metal structure at the plane of the blade through which the pin 13 passes to retain the structure 35 to the blade. This structure defines part of the radially inner surface of the duct 3. Each blade 10 is provided with a part 36 which prevents excessive angular movement of the blade about the pin by abutting the circular root. This restriction on the movement of each blade prevents it from interfering with adjacent engine parts when the fan is stationary.

Each blade of the fan rotor is twisted so that the camber and stagger angles have different values when mea sured at various stations along the blade. In this way the efficiency of the blade is improved. One consequence of this twisting of the blade, and the fact that the stacking lines 37 of successive blades diverge towards their radially outer ends, is that during rotation of the fan centrifugal force tends to deflect the blade. Because the stacking line of a blade is defined as a line drawn through the centre of mass of successive cross sections of the blade, centrifugal force will act on the blade so as to maintain the stacking line in a radial direction relative to the rotational axis of the fan rotor.

In a variable pitch fan engine in which two blades are mounted on a circular root, the root is rotated by approximately ninety degrees in order to reverse the thrust of the engine. Thus it will be seen that in the forward thrust position (FIG. 4) the tips 41, 42 of the stacking lines of the blades 43, 44 respectively, are separated in a radial plane by a greater distance than at their respective points of attachment 45, 46 to the circular root. The blade will tend to take up a position such that a line through points 42 and 46 passes through the axis 17. In the reversed thrust position shown in FIG. 5, the tips 41, 42 of the stacking lines of the blades 43, 44 are now separated in an axial plane, and points 42 and 46 are aligned again. Arrow A between FIGS. 4 and 5 indicates the direction of rotation of the rotor. Because centrifugal forces act radially outwards from the axis of the fan it will be seen that, in the absence of a pin root fixing, because of the mass distribution in each blade there is, in the reversed thrust mode, a considerable force acting on each blade trying to bend it about its point of attachment to the circular root. This force is present during most operating positions of the fan blades but at its worst as the blades pass through the position in which they are axially aligned. In the optimum reverse thrust position the blades may pass beyond the position in which they are axially aligned.

By providing a pin root fixing with the axis of the pin arranged perpendicular to the axis of the fan, each blade may lean towards the radial direction in all modes of operation so that bending moment induced in the blade can be minimised. During other operating positions of the fan blades the component of the bending moment that acts along the axis of pin is relieved and the stress at the point of attachment considerably reduced.

In addition to facilitating variation of pitch, putting two blades on a circular root reduces stress levels in the blade root by increasing the area thereof, and also reduces stress levels in the disc by avoiding the veenotches required with fir tree or dove tail root fixings.

We claim:

1. A multi-bladed fan comprising a rotor having aplurality of root members supported for rotation about respective axes extending radially in respect of the rotor, a pair of fan blades connected to each root member, pivot means connecting each fan blade of a said pair to its associated root member for pivotal motion about an axis transverse to said radial axis, each root member being rotatable through approximately between a first position in which the blades connected to the root member are aligned circumferentially and a second position in which the blades are aligned axially with respect to the rotor, and each of said fan blades is pivotable about said pivot means through a range in which each pair of blades can adopt an orientation in which their respective stacking lines are radially outwardly divergent by centrifugal force when the blades are aligned in accordance with said first position of the root member and can adopt an orientation in which their respective stacking lines are parallel by centrifugal force when the blades are aligned in accordance with said second position of the root members. 

1. A multi-bladed fan comprising a rotor having a plurality of root members supported for rotation about respective axes extending radially in respect of the rotor, a pair of fan blades connected to each root member, pivot means connecting each fan blade of a said pair to its associated root member for pivotal motion about an axis transverse to said radial axis, each root member being rotatable through approximately 90* between a first position in which the blades connected to the root member are aligned circumferentially and a second position in which the blades are aligned axially with respect to the rotor, and each of said fan blades is pivotable about said pivot means through a range in which each pair of blades can adopt an orientation in which their respective stacking lines are radially outwardly divergent by centrifugal force when the blades are aligned in accordance with said first position of the root member and can adopt an orientation in which their respective stacking lines are parallel by centrifugal force when the blades are aligned in accordance with said second position of the root members. 